This invention relates to buildup valves for use in internal combustion engines suitable for automobiles and the like.
Engine valves used in automotive engines including intake and exhaust valves are valves that control the intake of an air-fuel mixture and the discharge of spent gas in the combustion chamber. Each engine valve includes a valve head and a valve stem extending therefrom. The head of an engine valve is located within the combustion chamber. Cycles of tight engagement and separation are repeated between a valve seat snugly fitted in a cylinder head and the valve face. The valve face is required to be resistant to heat, corrosion, and wear because it is exposed to elevated temperatures of 700.degree. C. to 800.degree. C. in the combustion chamber and subjected to repetitive collision against the valve seat.
One prior art method for imparting heat resistance and other desired properties to the valve face of engine valves is by building up materials having excellent heat, corrosion, and wear resistances, for example, cobalt or nickel base superalloys on the valve face. Gas buildup techniques using acetylene gas are now prevalent for the building up of such materials on the valve face although TIG welding and plasma spraying may also be used.
Most these conventional engine valves have a buildup of at least 1 mm thick on the valve face for the following reasons. It is technically difficult to control the thickness of a buildup to a lower level by the acetylene gas buildup and TIG buildup techniques. The matrix material must be preheated before the deposition of a buildup material by the acetylene gas buildup and TIG buildup techniques. The preheating causes part of the matrix iron to melt so that iron is introduced into the buildup layer to dilute the buildup material, resulting in promoted deterioration of the buildup layer. In order to secure a sound buildup layer of a certain thickness, the initial buildup must be 1 mm or thicker.
The buildup materials are very expensive and cost, for example, 8,000 to 10,000 yen/kg. Because of the thickness and expensiveness of buildup material, the cost of buildup material occupies about 30 to 40% of the total cost of valve. An attempt was made to reduce the valve cost by reducing the thickness of buildup. In the conventional acetylene gas buildup and TIG buildup techniques, however, the overall valve face is hardened and thus become brittle during deposition of buildup material, which in turn, requires tempering. Therefore, simply reducing the thickness diminishes the resistance to deformation. In case of high performance, high power engines whose valve face is exposed to a high pressure, the valves themselves undergo considerable deformation and are thus unacceptable.